In an external pressure type hollow fiber membrane module in which a hollow fiber membrane bundle having about a few hundred to a several tens of thousands of hollow fiber membranes bundled together is accommodated in a cylindrical case, and both ends of the hollow fiber membrane bundle are adhered and fixed with resin, a water supply port for supplying raw water and a water discharge port for discharging are often arranged near both ends of the cylindrical case on a case side surface on the inner side than the adhered and fixed portion with respect to the axial direction of the cylindrical case. When the water supply port and the water discharge port are arranged on the case side surface, the device is structured such that a distribution cylinder (cylindrical body formed with great number of holes) is arranged on the inner side of the cylindrical case at a location where the water supply port and the water discharge port are arranged, and an annular flow path is formed to surround an outer periphery of the distribution cylinder, so that a dead space of the water flow is eliminated (see patent documents 1 and 2).
When raw water is supplied from the water supply port into the hollow fiber membrane module, the raw water passes through the holes of the distribution cylinder from an annular flow path near the water supply port and enters an outer side region of the hollow fiber membrane, and then penetrates through a hollow fiber membrane surface to immerse to the interior thereof, whereby the filtrate water passes through the interior of the hollow fiber membrane and is taken out from an opened end. When performing the raw water processing in a cross-flow method, the residual raw water that did not penetrate through the hollow fiber membrane surface passes through the holes of the distribution cylinder near the water discharge port, and passes the annular flow path to be discharged from the water discharge port.
In such hollow fiber membrane module, the raw water is supplied from the water supply port and the raw water is flowed out from the water discharge port, similar to the raw water processing in the cross-flow method, before starting the filter operation. The gas and the like that entered the module at the beginning then flows out from the water discharge port by supplying and discharging the raw water, so that the filter region in the module can be filled with raw water.
However, even if the interior of the hollow fiber membrane module is filled with raw water, if the filter operation is immediately started, the air remaining in a piping and the like in the middle of the raw water supply line is accompanied by the raw water and immerses in the hollow fiber membrane module with elapse of time, whereby the air may clog the hole of the hollow fiber membrane thereby reducing the effective filter membrane area. Therefore, even after the filter region is filled with raw water, the filter operation is not immediately started, and the raw water is continuously supplied and flowed out from the water discharge port for a while so that the air entered into the module can be discharged, and thereafter, the filter operation is started.
Furthermore, after the filtering process of a constant time is terminated, backflow washing of supplying filtrate water or high pressure air from the filtrate water exit side and flowing the same to the raw water side, and air scrubbing of supplying the raw water mixed with high pressure air of only the high pressure air from the water supply port side, and discharging suspended matters accumulated in the module are performed. The wash wastewater and waste gas are flowed out from the water discharge port even when performing such washing.
In such case, when the water or the air flowed into the filter region is to be discharged to the outside of the module, they pass through the holes (distribution hole) formed in the distribution cylinder to flow out to the annular flow path at a periphery of the distribution cylinder, and ultimately, flow out to the outside of the module from the water discharge port formed at a side surface. The water and the air passed through the holes of the distribution cylinder and flowed out all concentrate near the water discharge port, which increases the pressure loss near the water discharge port. As a result, the drive force necessary for the filter operation and the supply pressure in time of washing need to be raised, whereby loss of raw water also occurs in addition to the loss of time until the start of filtering and the loss of operation energy.
Such problems are not limited to a case where the water supply port is arranged near the end of the cylindrical case and at a case side surface position on the inner side than the adhered and fixed portion with respect to the axial direction of the cylindrical case, and similarly arise even when the water supply port is arranged at the end of the cylindrical case and the through-hole through which the raw water passes is formed at the adhered and fixed portion if the discharge port is arranged near the end of the cylindrical case and at the case side surface position on the inner side than the adhered and fixed portion with respect to the axial direction of the cylindrical case.
As a means for solving such problem, patent document 1 proposes changing the hole-opening area of the distribution hole formed in the distribution cylinder on the circumference so as to become larger the more distant from the discharge port, patent document 2 describes arranging the distribution cylinder such that the width of the annular flow path on the outer side of the distribution cylinder becomes a maximum near the water discharge port, etc.
The problem of increase in pressure loss due to increase in flow rate near the water discharge port can be considerably alleviated by the improved means described above.
However, a phenomenon in which the hollow fiber membrane is pushed by the flow of water and air trying to flow out, and the hollow fiber membrane is pushed against the vicinity of the distribution hole of the inner wall of the distribution cylinder occurs in the distribution cylinder, whereby the distribution hole tends to be easily blocked by the hollow fiber membrane bundle being pushed. The problem of blocking of the distribution hole by the hollow fiber membrane particularly easily occurs when the diameter of the distribution hole is small or when the filling rate is high or a great number of hollow fiber membranes are filled in the module. Thus, reduction of the pressure loss by the blocking of the distribution hole is demanded to sufficiently reduce the operation drive force in time of filter operation and the supply pressure in time of washing.
The above description is made in relation to the external pressure type hollow fiber membrane module, but in the case of an internal pressure type hollow fiber membrane in which the raw water supply port is arranged at a lower end and an upper end of the cylindrical case, the raw water is introduced to the inner side of the hollow fiber membrane and the filtering is performed from the inner side towards the outer side of the hollow fiber membrane, and the filtrate water is discharged from a filtrate water discharge port arranged in the vicinity of the end of the cylindrical case and on the case side surface on the inner side than the adhered and fixed portion with respect to the axial direction of the cylindrical case, the problem in that the hollow fiber membrane is pushed by the flow of water and air trying to flow out and is pushed against the vicinity of the distribution hole of the inner wall of the distribution cylinder thereby blocking the distribution hole and causing pressure loss similarly occurs.
The problem of blocking of the distribution hole in time of discharge is difficult to solve with the related art described above.
Patent document 1: Japanese Laid-Open Utility Model No. 62-190605 (Claims)
Patent document 2: Japanese Laid-Open Patent Publication No. 2004-50023 (paragraphs [0016] to [0017])